How this calculation works
Dew point describes how far an air parcel must cool before its vapor pressure reaches saturation. A cold window can reach that condition even while room air remains well above its dew point. The comparison needs the surface temperature as well as room humidity.
γ = ln(RH/100) + 17.625T/(243.04 + T); liquid-water dew point = 243.04γ/(17.625 − γ)A worked example
At 20 °C and fifty percent room RH, the entered formula gives a dew point of about 9.261 °C. A 10 °C surface is only about 0.739 °C above that estimate. Changing the surface to 8 °C leaves the room dew point unchanged but moves the surface below it. Lowering room humidity changes the vapor pressure itself, which is a different intervention from warming the surface.
Good to know
Liquid-water approximation and a static surface comparison only; no mold diagnosis, frost point, condensate mass or building approval. A surface below the threshold is a model condition, not a diagnosis of a leak or defective window. Consider measurements, local airflow and the actual envelope when investigating a real problem.
Reference: US EPA — Magnus relative humidity relation (opens in a new tab) ↗
What the model answers
Dew point describes how far an air parcel must cool before its vapor pressure reaches saturation. A cold window can reach that condition even while room air remains well above its dew point. The comparison needs the surface temperature as well as room humidity.
This page uses a liquid-water Magnus approximation. It does not estimate the glass temperature from outdoor weather, window ratings or room heat flow; supply a measurement or a separately justified thermal estimate.
The Layered wall thermal resistance calculator estimates boundary temperatures for an ideal series wall. Its thermal assumptions must be checked separately before using its surface temperature here.
Measure and enter consistently
Enter air temperature and relative humidity from the same location and time. A reading next to a radiator and a surface measurement in a remote corner may not represent the same local vapor conditions.
The surface input may be warmer or colder than the air. Near saturation, small sensor errors can change which side of the estimated threshold a surface falls on. The calculation reports a temperature margin so that closeness remains visible.
Interpret the result
A positive margin means the entered surface is above the approximate dew point. A negative margin means the uncondensed vapor would exceed liquid-water saturation there. It does not calculate how many drops form or how quickly they appear.
The surface saturation ratio can exceed one hundred percent because it is a hypothetical comparison before phase change. It should not be read as a stable measured indoor relative humidity. Below-zero results need a separate frost-point treatment for ice.
Work through the numbers
At 20 °C and fifty percent room RH, the entered formula gives a dew point of about 9.261 °C. A 10 °C surface is only about 0.739 °C above that estimate.
Changing the surface to 8 °C leaves the room dew point unchanged but moves the surface below it. Lowering room humidity changes the vapor pressure itself, which is a different intervention from warming the surface.
Where this model stops
Liquid-water approximation and a static surface comparison only; no mold diagnosis, frost point, condensate mass or building approval.
A surface below the threshold is a model condition, not a diagnosis of a leak or defective window. Consider measurements, local airflow and the actual envelope when investigating a real problem.
A few questions, answered
Why can the surface ratio exceed 100 percent?
It compares unchanged air vapor pressure with saturation at the colder surface. Actual condensation can remove vapor instead of sustaining that ratio.
Is dew point the same as frost point?
No. This relation uses saturation over liquid water and does not solve equilibrium over ice.